This application claims the priority of 196 30 219.6, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a brake master cylinder with a plunger piston acted upon by a brake linkage, with a floating piston which is arranged between two working spaces and separates the first and the second working space fluidically from one another.
Various configurations of a brake master cylinder are described in unpublished application Ser. No. P 195 43 698.9-21. In normal brake operation, these configurations permit decoupling of the brake pressure built up in the vehicle brake circuits by the actuation of the brake pedal. On one hand, the driver is provided with a sense of braking by way of a so-called pedal-travel simulator, in that the brake pedal operates against a corresponding counterpressure, and, on the other hand, a safety function is ensured in that, namely, in the event of failure of the electrics or hydraulics which provide the brake pressure in normal operation, the brake pressure built up by the brake pedal can flow out of the brake master cylinder into the brake circuits of the vehicle.
Such a brake master cylinder has a plunger piston which is acted upon by a brake linkage. A floating piston is arranged between two working spaces and separates the first and the second working space fluidically from one another. Each of the two working spaces open into a main brake circuit, and the floating piston is axially movable in the brake master cylinder.
German Offenlegungsschrift 2,026,756 provides, between the floating piston and the wall of the brake master cylinder, a sleeve which is axially movable in the brake master cylinder for the purpose of achieving a different object. The sleeve and the wall of the brake master cylinder each have an opening. This arrangement represents a multi-circuit brake system whose brake circuits are constructed independently of one another in order to allow the other brake circuit to function unimpaired thereby in the event of a failure of one brake circuit. The conditions and prerequisites for the movement of this sleeve and for the state in which the openings in the sleeve and the wall lie one above the other are fundamentally different from those described below in connection with the present invention.
It is an object of the present invention to provide for the improved functioning of a pedal-travel simulator.
To achieve this object, the present invention provides a brake master cylinder in which a sleeve is axially movable in the brake master cylinder and is provided at least between the floating piston and the wall of the brake master cylinder. The sleeve and the wall of the brake master cylinder each have an opening which, in interaction, connect the first working space, formed by the floating piston and the plunger piston, to a displacement passage at least in the case of a movement of the floating piston into a first position in the direction of a brake actuation. In the event of a movement of the floating piston beyond the first position, the sleeve is moved axially and separates the first working space from the displacement passage.
Given normal functioning of the brake system, the connection between the first working space and the displacement passage can be permanent or can be established only, when the brake is actuated, i.e. when the floating piston is moved into the first position, in accordance in that case with the normal functioning of the brake system.
An advantage of the present invention is that functioning can be achieved with comparatively little expenditure on components by using the sleeve in the brake master cylinder with corresponding seals.
In one advantageous configuration of a brake master cylinder, the sleeve is pressed against an abutment by a spring force, with the openings in the sleeve and the wall of the brake master cylinder interacting in this position. Furthermore, the sleeve and the floating piston are configured such that the sleeve is taken along by the floating, piston counter to the spring force when the floating piston is moved beyond the first position in a brake actuation direction.
With normal functioning of the brake system, the main brake circuits are separated from the brake master cylinder by controllable valves or the like in the closed position of the latter. Because of the volume (then closed off) of the second working space of the brake master cylinder, the second space being formed by the floating piston and the wall of the brake master cylinder, a comparatively high. pressure is built up in the second working space given a certain movement of the floating piston in the direction of a brake actuation when the brake is actuated. This pressure prevents a further movement of the floating piston in the direction of a brake actuation. In this situation, the first working space of the brake master cylinder, with the first working space being formed by the floating piston and the plunger piston, is connected to the displacement passage via the interacting openings in the sleeve and the wall of the brake master cylinder.
If there is a disturbance in the normal brake functioning, due, for example, to an electrical failure, the controllable valves between the brake master cylinder and the main brake circuits are in the open position. As a result, the pressure build-up in the second working space of the magnitude encountered with a closed controllable valve is prevented. The floating piston can thus move further and take the sleeve along in the process. This is achieved, for example, by positive engagement between the floating piston and the sleeve, with this positive engagement existing when the floating piston moves to a corresponding extent. The present invention also contemplates, however, that a spring with a suitably selected spring constant can be used to achieve this function. Because of the movement of the sleeve, the opening in the sleeve and the opening in the wall of the brake master cylinder then no longer interact. The result is that the first working space is no longer connected to the displacement passage. Thus, a brake pressure built up in the first working space with the movement of the plunger piston then propagates via the opened controllable valve into the main brake circuit assigned to this first working space.
In another embodiment according to the invention of a brake master cylinder, a sleeve which is axially movable in the brake master cylinder is provided at least between the floating piston and the wall of the brake master cylinder. The sleeve and the wall of the brake master cylinder each have an opening which, in interaction, connect the first working space formed by the floating piston and the plunger piston to a displacement passage at least in the case of a movement of the floating piston into a first position in the direction of a brake actuation. The sleeve is acted upon by a spring force, with the result that it is moved axially when the pressure in the second working space falls as a result of the opening of the valve of the main brake circuit assigned to this second working space. The first working space is separated from the displacement passage as a result of the sleeve movement.
With normal functioning of the brake system, the connection between the first working space and the displacement passage can be permanent or be established only when the brake is actuated, i.e. when the floating piston is moved into the first position, this then being in accordance with the normal functioning of the brake system. Control of the movement of the sleeve is performed directly by the pressure conditions in the second working space of the brake master cylinder. When the pressure in the second working space falls because of the opening of the controllable valve which connects this second working space to the associated main brake circuit, the sleeve is then moved as a result of the spring force, so that the first working space of the brake master cylinder is separated from the displacement passage.
In another advantageous configuration of a brake master cylinder of the above-mentioned type, the spring force to which the sleeve is subjected is supported on the plunger piston. This approach ensures that a movement of the sleeve in the sense of a separation of the first working space from the displacement passage takes place only when the brake is actuated. This proves particularly advantageous when the sleeve has at least one further opening, whereby one of the working chambers is connected to a brake fluid container via a corresponding opening in the wall of the brake master cylinder. Thereby, it can be ensured that the snifter opening is open when the brake is unactuated, allowing brake fluid to be balanced.
In another configuration of the present invention, a brake fluid container is connected to the displacement passage via a nonreturn valve. The nonreturn valve closes in the direction of the brake fluid container, and the opening in the sleeve is connected to the second working space in the position of the floating piston which corresponds to an unactuated brake. The opening in the sleeve then is sealed off from the first working space, and the opening in the sleeve is connected to the first working space in the first position of the floating piston, with the opening in the sleeve then being sealed off from the second working space.
Such a configuration is advantageous in that the opening in the sleeve and the opening in the wall of the brake master cylinder interact such that, with the brake unactuated, a connection is established between the second working space of the brake master cylinder and the brake fluid container. That is, the openings then act as a snifter opening for the second working space and, with the brake actuated, the first working space is connected to the displacement passage via the openings. These openings thereby exercise a twin function. Thereby, the present invention minimizes expenditure as regards sealing the openings.
In yet another configuration of the present invention, at least one further brake fluid container is connected to one of the two working chambers by at least one further opening in the sleeve, which interacts with at least one further opening in the wall of the brake master cylinder. The sleeve can be advantageously configured in a corresponding manner, so that the connection between the working spaces of the brake master cylinder and the brake fluid containers can likewise be established via openings in the sleeve.